The present invention relates generally to satellites, and more particularly to systems and methods used to determine the distance between an orbiting satellite and a ground station.
The assignee of the present invention develops the system architecture of satellite ground stations. The assignee also designs and constructs electronic circuits that communicate with satellites found in these ground stations. An important task of a ground station is to maintain the orbit of satellites. A necessary parameter used to maintain a satellite""s orbit is its altitude. A satellite""s altitude is commonly derived from measuring the distance or range a satellite is from the ground station. It is often necessary to determine the distance a satellite is from a ground station. The assignee of the present invention has developed an improved method for use in determining satellite range from a ground station.
Heretofore, the assignee of the present invention has used a technique that employs a method that is slow, un-flexible, introduces spurious signals, and provides the user with no quality factor, i.e., a measure of the certainty of its range number. This method is slow because it employs a single correlator that shifts and dwells through entire code sequences. It is un-flexible because it uses only two fixed code lengths. Introduces spurious signals because the final code sequences are composed of a series of short code sequences. Finally, it provides a range value without an accurate assessment of how certain the range value is.
In both methods (new and the old), the distance (range) to the satellite from a ground station is determined as follows. The ranging equipment constructs a long binary PRN digital ranging code. An important property of this code is that it does not repeat in the time required for it to be sent and subsequently received. This PRN ranging code is then modulated by uplink telemetry system at the ground station onto a carrier which is then transmitted to the satellite. This signal is then echoed back (retransmitted) to the ground station by the satellite. The received ranging signal is then routed via a downlink telemetry system to a receiver portion of the ranging equipment. Because the ranging signal is corrupted by noise and distorted by Doppler, a correlation process is performed. The correlation process multiplies the received PRN ranging code with a clean delayed version of the transmitted PRN ranging code. When the value of the delay is correct a large output of the correlator is observed (a correlation peak). When the value of the delay is not correct, the correlator yields a small signal value, or no correlation peak. The correct delay is equal to the time of flight of the signal. The distance to the satellite can be calculated based on this delay.
While the basic method of sending and then correlating the returned signal is the same for the old and new methods, it is the construction of the transmitted code and how the received signal is processed that differentiates them. It would therefore be desirable, and it is an objective of the present invention to provide improved systems and methods that determine the distance between an orbiting satellite and a ground station.
To accomplish the above and other objectives, the present invention provides for systems and methods that may be used to determine the distance between an orbiting satellite and a ground station. The present invention accomplishes this by (1) using a final PRN ranging code sequence that is a composite of three long PRN codes; (2) Simultaneously correlating the inner and middle code sequences; (3) Correlating a subset of the outer code; and (4) Deriving a quality value of the range number based on a running bit error rate (BER) on the outer code.
The advantages of using three long PRN codes to construct the final ranging code are flexibility, reduced processing time, and a spurious free composite signal. Flexibility is achieved by custom tailoring the lengths of the inner, middle, and outer codes for the specific orbit and signal power requirements of the ground station and the satellite. By simultaneously correlating the inner and middle codes the speed of the ranging process is greatly enhanced. Correlating a subset of the outer code further reduces the acquisition time. A spurious free ranging signal is derived from the property that long PRN codes evenly spread their energy across the frequency spectral band. Thus when the ranging signal is combined with other telemetry signals it does not have spurious content that could cause interference. The bit sequence of the outer code is unique, any errors are easily monitored. A running bit error rate based on the number of errors found in the outer code is the basis for a measure of the confidence factor the ranging unit assigns to the range number.
An exemplary system comprises a custom circuit board (or card assembly) and a central processing unit (CPU). The custom circuit board includes seven main sub-circuits. These sub-circuits include a master clock divider circuit, a transmit code generation circuit, a time tag circuit, a digitizing circuit, a digital synthesizer circuit, a matched filter circuit, and a field programmable gate array (FPGA) integrated circuit.
The master clock divider circuit supplies the custom circuit board with all the necessary clocks at the correct frequency and phase. The function of the transmit code generator circuit is to construct a final PRN ranging code. The time tag circuit marks the arrival time of each complete inner code cycle.
The received signal is digitized by an Analog to Digital Converter (ADC). The received signal is a composite signal that contains the PRN ranging signal and telemetry signals that are corrupted by noise and contain a Doppler offset and/or shift. The digital synthesizer circuit is used to acquire the received signal ensuring the ADC yields coherent samples. A frequency domain matched filter processes the digitized analog signal to produce the inner code correlation peaks. The FPGA circuit is used to select the correct data from the matched filter and transmit it to the central processing unit.
The central processing unit comprises a middle code software matched filter that generates an outer code bit. The final location of the received code is known once seven bits of the outer code are processed. The central processing unit computes the distance from the ground station to the satellite by calculating the difference between the time that the composite signal was received compared to the time that the composite signal was transmitted and dividing the difference value by the speed of light. The central processing unit also tracks any errors found in the outer code and produces a bit error rate signal indicative of the validity of the computed range value.
An exemplary method for determining the distance between an orbiting satellite and a ground station comprises the following steps. Inner, middle and outer pseudo-random number codes are generated. The PRN ranging code is the composite of the inner, middle, and outer codes. The PRN ranging code is then transmitted from a ground station to a satellite whose range is to be determined. The analog signal is retransmitted from the satellite to the ground station.
The analog signal is digitized at the ground station. The digitized analog signal is match filtered to produce inner code correlation peaks. The middle matched filter then performs a correlation process on the output of the inner matched filter. The middle code correlation peaks are match filtered to determine a bit of the outer pseudo-random number code. A predetermined number of consecutive outer pseudo-random number code bits are match filtered to determine the position of the outer code in the composite code. The distance from the ground station to the satellite is determined by calculating the difference between the time that the code sequence was received compared to the time that the code sequence was transmitted.
The present invention provides for simultaneous correlation of the entire inner and outer code sequence. The present invention incorporates a matched filter early/late gate that acts as a digital phase detector. The present invention has a flexible range code to adapt to varying orbit distances, low signal to noise ratio environments, Doppler shifts, and changing Doppler shifts. The present invention also uses an applied bit error rate algorithm to give a confidence value to the range number (using the outer code).
The present invention has the ability to optimize performance of system by changing (inner, middle, outer) code(s) to adapt to all types of satellite orbits. The present invention has reduced acquisition time by using a matched filter fast Fourier transform (FT) correlator engine to simultaneously correlate the entire inner and middle code sequence verses the currently-used architecture discussed in the Background section. The output spectrum generated by the present invention minimize s interference with other signals due to the fact that ranging power is spread over the entire bandwidth of operation, similar in concept to CDMA (carrier detect multiple access).
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing, wherein like reference numerals designate like structural elements, and in which:
FIG. 1 shows the operating environment of ranging systems and methods in accordance with the principles of the present invention;
FIG. 2 is a block diagram showing the architectural topology of exemplar y digital ranging equipment in accordance with the principles of the present invention that may be used to implement the present digital ranging systems and methods;
FIG. 3 is a block diagram showing the architectural topology of the inner code matched filter correlator engine shown in FIG. 2; and
FIG. 4 is a flow diagram showing an exemplary ranging method in accordance with the principles of the present invention.